# Patent

## Title
Polythylene aerogels and method of their production

## Abstract

A method for making polyethylene aerogels, including high molecular weight aerogels, commences by in a hydrocarbon solvent (e.g., toluene or benzene) in which polyethylene is insoluble at room temperature, adding polyethylene to the solvent heated to a temperature at which said polyethylene is soluble to form a reaction mixture. A free-radical catalyst is added to the reaction mixture and the reaction mixture is held until the desired gelation of the polyethylene has been achieved. The reaction mixture is cooled to about room temperature; and the hydrocarbon solvent is replaced with a gas (e.g., CO2 or air) to form the polyethylene aerogel. Optionally, the cooled reaction mixture can be contacted with an anti-solvent for polyethylene to replace the hydrocarbon solvent with the anti-solvent. Silica aerogel particles can be encapsulated in polyethylene aerogel by adding the particles to the polyethylene gelation reaction mixture.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims benefit of U.S. provisional application Ser. No. 61/491,384, filed on 31 May 2011.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND
Aerogels are three-dimensional network of nanophase architecture composed of gas [usually air] and solid structure, with air [gas] to solid ratio of up to 99% by volume. Thus aerogels have high porosity, high specific surface area, and very low density. The entrapped motionless air [gas] in the pores is responsible for the high insulation [low thermal conductivity] of the aerogel materials. Aerogels can be made as large monolithic shapes, or micron/submicron size bodies, or any size and shape in between. The gas inside the aerogel nanostructures can be replaced [displaced] with other substances, such as, for example, pharmaceuticals, which then are released into the desired targets in a controlled manner.
Polyethylene is a non-polar material with many desirable properties. At ambient conditions, it is considered rather chemically inert, non-toxic, transitive of infra-red [IR] radiation, outstanding electrical insulating properties, such that its dielectric constant, and power factor are almost independent of frequency and temperature. It has also desirable mechanical properties.
Research, conducted to explore the possibility of preparation of novel polyethylene only aerogel materials, managed to create a novel methodology to manufacture polyethylene aerogels, which included preparation of polyethylene aerogels using standard polyethylene polymers [low and medium molecular weight (MW)] as aerogel precursors. Research proceeded to further develop this material to comply with multiple specifications [including radiometric, mechanical, thickness, as well as several other requirements]. It was then realized that preparation of thin [2-3 mm] and ultrathin [0.5-0.9 mm] aerogel sheets (using the standard polyethylene polymers), which is necessary for packaging of electronics, compromised radiometric and mechanical properties of the aerogel sheet. To solve this shortcoming, the research efforts obtained high density [HDPE] and ultrahigh molecular weight [UHMW] polyethylene polymeric precursors and proceeded to develop polyethylene aerogel sheets from these precursors to meet all the specifications and performance requirements for electronics and sensors applications.
These newly developed polyethylene aerogels may be considered as a platform technology for many other applications, including thermal, cryogenic, and sound insulation; sound frequency modulation, computer electronics, wide band antennas, membrane separations, filtration, as well as controlled release of encapsulated substances. Several product specifications, and performance criteria, such as mechanical, radiometric, insulation, and suitability for packaging of electronics, were met with outstanding results.
BROAD STATEMENT
A method for making polyethylene aerogels of high molecular weight commences by in a hydrocarbon solvent (e.g., toluene or benzene) in which polyethylene is insoluble at room temperature, adding polyethylene to the solvent heated to a temperature at which said polyethylene is soluble to form a reaction mixture. A free-radical catalyst is added to the reaction mixture and the reaction mixture is held until the desired gellation of the polyethylene has been achieved. The reaction mixture is cooled to about room temperature; and the hydrocarbon solvent is replaced with a gas (e.g., CO2 or supercritical CO2) to form the polyethylene aerogel. Optionally, the cooled reaction mixture can be contacted with an anti-solvent for polyethylene to replace the hydrocarbon solvent with the anti-solvent. Silica particles can be added to the initial reaction mixture.


DETAILED DESCRIPTION
Desired Polyethylene Aerogel Properties Include:


    1—High thermal insulation [low thermal conductivity; in the range of 5-45 mW/m-K];
    2—Integral and uniform micro and macro physical structure with adequate mechanical strength for handling;
    3—The ability to produce very thin sheets [1-3 mm], and ultra-thin sheets [0.1-1.0 mm] encompassing 1 & 2 above;
    4—Good radiometric properties, such that high radiation transmission through the aerogel is not impeded by reflection, scattering, or absorption of such radiation;
    5—Ability to reversibly stick to other materials or other aerogel sheets in electronic packaging.
    6—High electric sheet resistance.

Aerogel Preparation Strategy:

1—Gelation Mechanism:



Polyethylene is soluble in hot hydrocarbon solvents, such as, for example, toluene and benzene, but insoluble at ambient room temperature. The dissolution temperature of polyethylene in toluene increases with the starting polymer molecular weight. Dissolved polymer molecules in solution are then bridged together to form a three dimensional network that entrap the solvent inside them [i.e., liquid gel], which upon cooling to ambient temperature, becomes solid gel of polymer network enclosing liquid toluene solvent inside the pores. On removal of the liquid solvent from the gel network, the solvent is replaced with a gas [air or CO2 gas for example] and forms a dry polyethylene aerogel.
Polymer gelation bridging is accomplished by a peroxide catalyst, such as, for example, benzoyl peroxide, that causes free radical reactions between the polymer molecules, when activated by heat or ultraviolet radiation energy. The overall reactions are believed to be as follows:

    1—Peroxide catalyst activation:

ROOR[e.g., benzoyl peroxide]+heat or UV light=2RO.[active free radical catalyst]

    2—Initiation/polymer activation:

Polymer solution(2P)+Peroxide(2RO2.)=2P.+2RO+H2(gas)

    3—Polymer-polymer bridging—Free radical termination:

2P.=(P—P)n bridged polymer network

    4—Catalyst decomposition:

2RO+heat[or UV light]=2CO2(gas)+2Benzene

    5—Hydrogen consumption:

H2+0.5O2(air)=H2O.

    6—Overall polymer-polymer bridging gelation Reaction:

ROOR+2P+heat[or UV light]=(P—P)n polymer gel+CO2(gas)+H2O+2Benzene.

    7—Gelation reactions by-products, benzene, water are removed from gel during supercritical drying, while CO2 gas is released to atmosphere during gelation.


The duration of the active free radical catalyst is determined by the number of half life cycles [half life is the time required to decompose 50% of remaining catalyst in solution]. It is believed that 97% of benzoyl peroxide is decomposed after five (5) half life cycles, and 99% decomposed after (7) seven half lives. Half-life temperatures are: 10-hours at 73 C.°; 1-hour at 92 C.°; and 1-minute at 131 C.°. Thus, at a gelation reaction temperature of 130 C.°, 99% of catalyst is decomposed after about 7 minutes of reaction.
2—Thermal Insulation & Radiation Absorption:
Production of low-density aerogel would achieve both (a) low absorption of IR radiation, since absorption is a function of weight, (b) as well as increase thermal insulation due to higher pore volume/volume porosity. The goal of thermal insulation performance of these aerogels is to achieve a thermal conductivity, [K] in the range of 5-45 mW/m-K°.
3—Mechanical Strength:
From gelation equations, control of humidity in air and solvents and chemicals involved in the gelation process is vital to achieving more complete gelation linkages and better mechanical strength. Also, higher molecular weight polymers would afford more linkages and lead to better mechanical strength aerogels.
4—Thin and Ultra-Thin Aerogel Sheets:
These would be possible if 2 & 3 above are achieved.
5—Reflection and Scattering Minimization:
Reflection of radiation by polyethylene aerogels dried at pressures lower than 1300 psi was minimized by the use of solvent exchange with acetone, an anti-solvent for polyethylene. Polyethylene aerogels treated with the acetone anti-solvent prior to drying had excellent radiometric performance, e.g., very low reflectance, compared with those not treated with anti-solvent.
However, it was observed that improved radiometric characteristics of the 2 million MW polyethylene aerogels also was achieved by implementing better supercritical drying techniques at higher pressures [over 1500 psi] and lower temperatures as shown in Table 8.
A possible mechanism of lowering reflection of polyethylene aerogel using anti-solvent is believed to be through formation of nano-sized “bumps” or precipitates on the aerogel structure. These nano-size “bumps” are smaller than the light's wavelength, so that the surface acts as having a continuous refractive index gradient between air and the aerogel, which decreases reflection by effectively removing the air-aerogel interface.
Another possible factor is the removal of toluene from the gel through solvent exchange with acetone, and higher solubility of acetone in supercritical CO2 fluid, leading to more thoroughly dried aerogel with minimal toluene residues.
6—Reversible Adhesion and Shrinkage Elimination of PE Aerogels:
This was a chance discovery from enhanced supercritical carbon dioxide drying of gels using higher pressure than “standard” practice, which also minimized or eliminated shrinkage of gels during drying.
DETAILED PREPARATIONS AND EXAMPLES
Procedure for Polyethylene Aerogel Production/Preparation
Preparation/production of polyethylene aerogels involves the following tasks:
1—Preparation and casting of gel solution.
2—Removal of excess free solvent from gel [optional].
3—Anti-solvent treatment of gels [optional].
4—Removal of entrapped solvent from gel to produce dry aerogels.
1—Preparation and Casting of Polyethylene Gel Solution:
1-1: Polymer Dissolution:
In a typical preparation procedure, polyethylene solid powders or beads are dissolved in hot toluene solvent under reflux while being mechanically stirred in a glass pressure reactor. Electric heating tapes wrapped around the outside of the reactor and covered with an insulating sheeting material heat the reactor used in the laboratory preparations. The heating tape was controlled by a temperature controller, which was set at a predetermined temperature. Dissolution temperature was higher for higher molecular weight [MW] polymers and was around 130 C.° for 2-6 million MW. Dissolution was continued until a clear polymer solution was obtained. The amount of polymer dissolved varied from less than 1% wt. for ultrahigh molecular weight [UHMW] to 10% or higher for low MW polymers, depending on desired final aerogel properties [for example, density & porosity] to be achieved.
1-2: Addition of Gelation Catalyst:
Gelation catalyst used in laboratory preparations was benzoyl peroxide, which was supplied at about 50% wt. powder mixture with phthalate to avoid explosive hazard during shipping by air. Warm/hot benzoyl peroxide solution was made by dissolving predetermined amounts in toluene and heated to a warm temperature. The amount of benzoyl peroxide was varied between 2-20% of the polyethylene polymer weight, but it was selected at the equivalent of 10% wt [20% including the phthalate powder] of the polyethylene weight for almost all of the preparations, unless stated otherwise. The catalyst solution was added to the clear polymer solution while mixing was continued under reflux at set temperature for 10 minutes, unless otherwise stated. The reaction product was the gel solution, which remained clear at the reaction temperature.
1-3: Casting Gel Solution:
Hot gel solution was poured out of the reactor by opening the reactor bottom valve into heated and leveled metal (stainless steel plate on aluminum base) or glass molds. All molds had glass covers with a center hole to allow reactor valve tube access to mold but keep hot solvent vapors from escaping. All heating and mixing units were switched off and gel was allowed to cool down in mold to room temperature. On cooling gel converted from solution to solid wet gel.
2—Removal of Free Excess Solvent from Gel:
Small amounts of excess free solvent that are not part of the gel structure [entrapped inside gel network] are removed from cooled gel body/sheet in the laboratory preparations by one or more of the following methods:
(a) Draining of solvent from casting mold;
(b) Partial air-drying at ambient conditions [slow evaporation];
(c) Partial forced air-drying at elevated temperatures [fast evaporation];
As discussed later, the need for this step may not be needed in future, as improvements in gel solution preparation are achieved to minimize or eliminate the presence of excess free solvent.
3—Anti-Solvent Treatment of Gels:
In an effort to improve the IR-radiometric properties of the polyethylene aerogels with regards to minimizing radiation reflection, the wet gels were treated with an anti-solvent. In this case, the anti-solvent of choice was acetone. Thus, the toluene-loaded gels were either transferred to and soaked in a bath of acetone for a period of time [0.5-2.0 hours], or sprayed in mold with acetone and kept soaking for a period of time. The resultant gels became snow white, and were taken to the final drying step.
4—Removal of Entrapped Solvent from Gel to Produce Dry Aerogel:
Wet gels were dried by both subcritical methods including ambient and forced air drying, and supercritical carbon dioxide solvent exchange in an autoclave. Tables 1-4 show both the aerogel preparation conditions and the resulting characteristics of those aerogels. The role of supercritical drying [and suggested further improvements] on the various properties of polyethylene aerogels are discussed later.
Results and Discussion:
Production of Polyethylene Aerogels:
Tables 1-8, show that polyethylene aerogels can be prepared by the methods described herein, with a range of performance characteristics. For example: aerogel density ranges from 0.6 g/cc to 0.01 g/cc or less; volume porosity range from 40% to over 99%; pore volume from 0.86 to over 80 cc/g; thickness from over 10 mm to less than 0.3 mm and all the way down to 0.007-0.028 mm [Gossamer aerogels]; mechanical properties from brittle, to tough, to very flexible; radiometric from unacceptably high reflectance, absorption, and scattering to very low values and excellent performance.
Additional properties achieved by discovery include polyethylene aerogel sheets with silky smooth feel and strong but reversible static cling, which obviated the need to use adhesives/glues to stick layers of sheets materials together in the packaging of electronic and sensor assemblies and, thus, improving overall performance and costs.
In achieving these results, many parameters and variables were identified, investigated and in many cases optimized. The main example parameters include:
1—Role of Humidity on Mechanical Strength of Polyethylene Aerogel:
Recognition of the possible deleterious effect of relative humidity in laboratory air and in solvents and chemicals used in preparation was made in early work with several attempts to add dehydrating reagents to ingredients in preparations but without much success. Finally, in later work, the use of anhydrous toluene, chemicals, and keeping laboratory air to below 40% [30-35%] at moderate room temperatures below 26 C.°. By avoiding humidity, the aerogels became much less brittle.
2—Role of Precursor Polymer Molecular Weight:
The results show that high [up to 1 million] and ultra high [2 million and above] polymer molecular weights are instrumental in improving mechanical strength, and making possible preparation of very thin aerogel sheets. In addition radiation scattering was improved.
3—Role of Anti Solvent on Radiometric Performance:
A chance discovery based on a hypothesis that polyethylene reflectance can be reduced by treating the wet toluene-laden gel sheets [or other shapes] in polyethylene anti-solvent, such as acetone, to exchange with and replace toluene in the gel structure. The first observation in the laboratory that acetone had an “opaquing, reflection lowering” effect on polyethylene [2 million] was made when the wet gel sheet turned “snow white” on soaking in acetone bath for 1-2 hours. However, the first successful preparation of 2M polyethylene aerogel sheet was made in later research, where marked improvement in aerogel reflection was made, compared with same aerogel made in earlier work, which was not treated with anti-solvent fluid exchange. All polyethylene aerogels thereafter were treated with acetone anti-solvent before drying with supercritical carbon dioxide. Table-5 shows examples of radiometric properties of polyethylene aerogels with and without anti-solvent treatment.
4—Role of Aerogel Density:
As expected, the results show that lowering aerogel density lowered radiation absorption, improved pore volume and porosity and, thus, improved insulation performance as well.
5—Role of Gel Drying Method:
Subcritical & Ambient Drying:
Table 1 A & B, show that subcritical and air drying can produce aerogels with a density range from 0.3-0.6 g/cc and a porosity range from 52% to 75%. Although subcritical including air drying methods can be used to completely remove the entrapped solvent to produce aerogels with adequate insulation performance, the micro physical structure is often non-uniform, and the mechanical and radiometric properties are far inferior to gels dried by supercritical carbon dioxide. Therefore, supercritical carbon dioxide solvent exchange was the method of choice for obtaining superior aerogels.
Drying with Supercritical CO2 Fluid:
Supercritical drying did not automatically always yield expected results, however. It was thought that as long as supercritical conditions are employed, drying to perfect aerogels would result. Instead, gel shrinkage and other unexpected performance problems occurred. In later work, then, attention was paid to controlling the supercritical drying parameters, such as pressure, temperature, and continuous [dynamic] flow rate and time of supercritical carbon dioxide after the autoclave has achieved supercritical conditions. Thus, by increasing the pressure to 1400-1450 psi, at 35-37 C.°, and supercritical CO2 flow for 20-30 minutes, no more shrinkage and superior products of 2M polyethylene aerogels were obtained. A similar observation on the beneficial use of higher pressure [1390 psi] than the normal 1250 psi with regards to shrinkage was made in earlier work, but its importance was not fully recognized at that time.
Relationship of Solubility of Solvents and Other Substances with Density of Supercritical Carbon Dioxide Fluid:
The importance of controlling supercritical pressure and temperature can be better understood by realizing that solubility of solvents, such as toluene, acetone, and alcohols, in supercritical carbon dioxide fluid is controlled by the density of the supercritical fluid, which is controlled by both the pressure and temperature. Therefore, better drying would be expected at the higher pressure and the corresponding temperature that enable the highest density of supercritical carbon dioxide [closer to the maximum of 0.8 g/cc].
Also, the amount of supercritical carbon dioxide fluid to completely dissolve the solvent [e.g., toluene, or acetone] is determined by the solvent's solubility [wt. %], and sufficient amount of supercritical CO2 should be allowed to flow to achieve this.
Effect of Pressure and Temperature on Density of CO2 Supercritical Fluid:
Table 6 shows the variation in density of supercritical carbon dioxide with pressure and temperature. These data were extracted from the CO2 density-pressure phase diagram. From Table 6, it can be seen that in the pressure range 90-110 bar [1 bar=14.75 psi] there is almost a linear relationship between the change in density with pressure and temperature, while outside this pressure range, it is a non-linear relationship.
Effect of Pressure:
The density of CO2 supercritical fluid (ΔD) increases with increasing pressure (ΔP) as follows:

ΔD/ΔP=0.007±(0.002)g/cc/bar;

Thus for 10 bar increase, the density increase is 0.07 (±0.02) g/cc.

Effect of Temperature:

The density of CO2 supercritical fluid decreases with increasing temperature (ΔT) as follows:

ΔD/ΔT=0.0164(±0.0018)g/cc/C.°.

An Empirical correlation between substance solubility change (ΔS) with a change in pressure and temperature of supercritical CO2 fluid is as follows:

S2=1.7×(d2/d1)×S1  [1]

where S2=Solubility at the higher pressure, S1=Solubility at the lower pressure, d2=Density of CO2 at the higher pressure, d1=Density of CO2 at the lower pressure.

Conversely, solubility at the lower pressure in relation to that at higher pressure is as follows:

S1=0.59×(d1/d2)×S2  [2].

In each case, the density is calculated to account for the effects of both the pressure and temperature.

6—Formation and Removal of Excess Free Solvent from Gel:

The presence of excess free solvent [not included in gel structure] is believed to be due to:

    A—Use of reflux condenser during gelation, that returning condensed solvent [toluene] is excluded from participating in gel structure, and,
    B—Melting polymer gel at inside surfaces of reactor thereby releasing included solvent from its structure. Melting of polymer gel is a result of inefficient heat transfer from heating tape wrapped around reactor where the temperature is higher than the interior of gel solution, where the temperature is controlled. The amount of polymer lost for melting inside reactor is a function of gelation time, as shown in Table 7. Approximately 20-29% of the initial polymer is lost to melting in 10 minutes of gelation, while, 54-61% is lost during 30 minutes, and about 65% is lost during 60 minutes of gelation time.

Removal of excess free solvent is made by partial air-drying methods as explained earlier. However, to prevent the formation of excess solvent in the first place, one or more of the following steps should be taken:

    1—Design better heat transfer mechanism, such as circulating temperature controlled heating oil around reactor;
    2—More efficient mixing to transfer heat from outside of reactor to interior of gel solution;
    3—Employ shorter gelation time inside reactor, but continue gelation reaction in the heated casting mold;
    4—Convert reflux condensation to a distillation function during gelation period, by letting evaporated solvent to condense outside reactor into a special container, but stop distillation before gel solution becomes more viscous than desired for casting;
    5—Reduce heating temperature during gelation to 70-80° C. to avoid excessive evaporation of solvent but only as long as polymer gel solution remains clear without polymer precipitation.









TABLE-1-A







Preparations of “Low Density” Polyethylene 


Aerogels [MW: K = 1000, M = 1 million]












Prep-

Polymer
Reaction




aration

Conc., 
Temp., 
Drying



Date
Polymer MW
wt%
C.
Method
Remarks















11/24/
135K
10
75
Air dry



11/23/
135K
5
75
Air dry +







subcritical 







CO2 @







54 C. &







670 psi



10/10-19/ 
135K
5
110
Air dry +







supercritical







CO2 @







1350 psi & 







52 C.



10/10-19/ 
35K (a)
5
110
Same as







above



10/10-19/
35K (b)
5
110
Air dry only



6/27/
135K + 4K 
10
130
Supercritical




[80:20]


CO2 @ 1310







psi & 52 C.



6/27/
135K + 35K 
10
130
Same as




[80:20]


above



6/27/
135K + 
10
130
Same as




3035K + 304K


above




[80:10:10]






9/28/
135K
10
134
Air dry,
3′′ x 10′′






desiccator
sheet, 







brittle


9/30/
135K + 
10
130
Same as
[repeat



35K + 4K


above
6/27/]



[80:10:10]
















TABLE 1-B







Performance Characteristics of Preliminary Polyethylene Aerogels

















Prepn. 
Polymer

D/L ×
ρa
Vp
Pv






Date
MW
mm
W cm
g/cc
cc/g
%
[K] 
[M]
[R]
Remarks




















11/24/
135K
6-9
3.8
0.46 
1.45
63.5
20
G/U
U
Cracks, off white


11/23/
135K
5.7
5.7
0.30 
2.49
74.9
20
G

white, no cracks


10/10-19/
135K
6
5.8
0.12
7.65
90.24

G




10/10-19/
35K (a)

3.4
0.26 
2.96
78.05

G




10/10-19/
35K (b)


0.62
0.84
51.83






6/27/
135K + 4K
8.9
4.5
0.27 
2.87
77.5

U
U
poor



[80:20]











6/27/
135K + 35K
10
4.5
0.2
4.3
83.75

U
U




[80:20]











6/27/
135K + 35K +
12
4.9
0.2
4.17
83.3

U
U




4K












[80:10:10]











9/28/
135K
10
7.6 × 25.4
0.17 
5.19
86.17

U

brittle





cm









9/30/
135K + 35K +
10
5.7
0.17 
4.95
85.6

U
U
Broke to pieces



4K;












80:10:10





Insulation: [K] = Thermal conductivity [mW/m-K°]; Mechanical [M]& Radiometric [R] performances: Good = G; Unsatisfactory = U, Excellent = E. Pore Volume, Vp , Volume Porosity = Pv , Apparent Density = ρa, Thickness = Th, Dimensions: D = Diameter: Length × Width = L × W.













TABLE 2A







PREPARATION OF HIGH DENSITY AND ULTRA HIGH 


MOLECULAR WEIGHT [UHMW] POLYETHYLENE AEROGELS















Polymer
Solution

Anti-



Preparation
Polymer
conc., 
Temp.,
Drying
solvent



Date
MW*
wt %
C.
Methods
treatment
Remarks
















9/11-14/
300K
5
110
CO2
No
dehydrating






Supercritical

agent used,






@ 1280 psi &

washed before






49° C.

drying


9/11-14/
300K
5
110
Same as
No
No dehydrating






above

agent added


10/2/
300K
5
176 F.
CO2 subcritical
No







@ 970 psi &








63 C.




10/2/
2M
5
135
Same as
No
Some shrinkage






above

in desiccator


10/3/
6M
5
135
CO2
No







supercritical




11/9-12/
2M
5
135
Supercritical
No
~50% shrinkage



[3′′ × 10′′]
















TABLE 2B







Aerogel Characteristics & performance














Preparation
Polymer
Th,
D/L × W 






Date
MW
mm
cm
ρa, g/cc
Vp, cc/g 
Pv, % 
Remarks

















9/11-14/
300K
6-8
4.2
0.075
12.50
93.75
Good white









monolith


9/11-14/
300K
6
3.2
0.097
9.48
92.0
Good white









monolith


10/2/
300K
10
4.9
0.096
9.58
92.0
good


10/2/
2M
5
3.7
0.34
2.11
71.74
Good, shrinkage,









yellowish


10/3/
6M
13
4.7
0.124
7.23
89.67
Very good


11/9-12/
2M
3
13.6
0.284
2.69
76.3
Intact, pliable





Pore Volume = Vp, Volume Porosity = Pv, Apparent Density = ρa, Thickness = Th, Dimensions: Diameter = D; Length × Width = L × W.













TABLE 3A







PREPARATION OF ULTRAHIGH MOLECULAR WEIGHT POLYETHYLENE AEROGELS















Polymer






Preparation
Polymer
conc.,
Solution
Drying
Anti-



Date
MW
wt %
Temp., C.
Methods
solvent?
Remarks
















12/10-11/
2M [3 × 10′′]
5
126
supercritical
No
Shrinkage






+ air dry

[7.75 × 2.5′′]


7/18/
2M [4 × 12]
2.5
126
Subcritical
No







@ 770 psi




9/6-11/
2M
2.8
130
Supercritical
No
Almost No






@ 1380 psi

shrinkage






& 56 C., 2








runs




10/15-19/
2M [19 × 8′′]
2.1
128
Air +
Yes
Transparent


*Anti-solvent



supercritical
[ 30 min]
gel became


treatment





white when


*Anhydrous





soaked in anti-


Toluene use





solvent.








Shrinkage only








after air dry








to [10 × 5′′]


12/1-5/
2M [25′′ × 4′′]
2.0
130
Air dry +
Yes
Shrinkage by






supercritical

air drying


5/7-8/ 
2M [4 × 12′′]
2.14
130
Air dry +
Yes







supercritical




6/9-11/ 
2M [4 × 12′′]
1.7
126
Air dry +
Yes







supercritical








@ 1280 & 








42 C.




7/15-17/
2M [4 × 12′′]
0.96
129
Air dry +
Yes
Excellent very






supercritical

small






@ 1290 & 

shrinkage






45 C. + scCO2








flow 6








minutes




7/21-22/
6M [4 × 12′′] 
0.95
129
Air dry +
Yes
Excellent no






supercritical

shrinkage






@ 1290 & 








45 C. + scCO2








flow 6








minutes




8/5-6/ 
6M [4 × 12′′] 
1.2
130
Air dry +
Yes
Very good






supercritical








@ 1290 & 








45 C. + scCO2








flow 6








minutes
















TABLE 3B







Aerogel Characteristics & Performance














Preparation
Polymer


ρa





Date
MW
Th, mm
L × W cm
g/cc
Vp, cc/g
Pv, %
Remarks

















12/10-11/
2M
7
19.7 × 6.35
0.195
4.3
83.79
Very good



[3 × 10′′]








7/18/
2M
25
21.65 × 6.5 
0.3
2.5
75
good



[4 × 12′′1








9/6-11/
2M
17
4.5
0.34
2.11
71.86
Good, but


* No Anti-






high


solvent use






reflectance


10/15-19/
2M
4
24.13 × 6.35 
0.417
1.57
65.26
Very good


*Anti-
[19 × 8′′]





mechanical


solvent






and


treatment






improved


*Anhydrous






radiometric


Toluene use






properties.









Lower









reflectance


12/1-5/
2M
3.5
24.77 × 11.43
0.40
1.67
66.68
good



[25′′ × 4′′]








5/7-8/
2M
2.8
21.5 × 6.2 
0.26
3.01
78.34
Some



[4 × 12′′]





shrinkage,









good









mech., not









uniform


6/9-11/
2M
1.0
27.5 × 9  
0.097
9.17
91.67
Very white,



[4 × 12′′]





uniform,









flexible,









best to









date


7/15-17/
2M
4
 27 × 8.5
0.043
22.43
96.44
Excellent,



[4 × 12′′]





flexible


7/21-22/
6M
2
29.6 × 7.9 
0.037
26.2
96.94
Excellent



[4 × 12′′]








8/5-6/
6M


0.047 
20.45
96.10
Very good



[4 × 12′′]
















TABLE 4A







Preparation of Very Thin And Ultrathin Polyethylene Aerogels















Polymer






Preparation
Polymer
Conc.,
Solution
Drying
Anti-



Date
MW
wt %
Temp., C.
Method
solvent?
Remarks
















8/12-14/
6M
1.06
130
Air [18 hr.] +
Yes




[0.9 mm]


supercritical




8/25-28/
6M
1.3
131
Air +
Yes




[0.8 mm]


autoclave +








flow




10/9-10/
6M [Std]
1.05
130
Same as
Yes







above




10/9-10/
6M [skin]
1.05
130
Same as
Yes
Short air






above

drying [<1 hr.]


4/6-8/
2M
0.71
133
Air [12 hr] +
Yes [2 hr]







supercritical




4/13-16/
2M
0.7
127
same
Yes
Squeeze Gel








between








2 glass








sheets


4/14-16/
6M
0.71
130
Air [7 hr] +
Yes
Squeeze gel






supercritical




4/21-23/
6M
0.71
130
Air [7 hr] +
Yes
No squeeze






supercritical




5/26-27/
2M
0.71
130
Air [1 hr
Yes
Squeeze



[Gossamer]


warm hair








dryer + 1 hr








ambient] +








supercritical




6/30-7/2/
2M [std]
1.09
128
Air [5.5 hr] +
Yes
Some






supercritical

shrinkage


7/7-10/
2M [std = 
0.9
129
Air [7.5 hr] +

No shrinkage


*Higher
2.5mm]


supercritical




pressure,



@ 1400




low temp



psi & 35 C.




[high








density of sc








CO2]








7/14-18/
2M [std]
0.9
129
Supercritical
Yes
Excellent No






@ 1400 psi

shrinkage






& 37 C.




8/26-28/
2M
0.76
129
Air [1 hr] +
Yes
Thin sheets






supercritical

stick to








supports


9/21-23/
2M
0.48
129
Air [40 min] +
Yes [40
Simultaneous






supercritical
min]
acetone








spray and air








dry, no








squeeze,








stick to








support


10/7-9/2009
2M
0.29
129
Air [1.5 hr] +
Yes







supercritical


















TABLE 4B







Characteristics and Performance of Polyethylene Aerogels



















Pore




Preparation
Polymer
Thickness
Dimension
Density
volume
Porosity



Date
MW
Th, mm
L × W cm
ρa, g/cc
Vp, cc/g
Pv, % vol
Remarks

















8/12-14/
6M
0.94
29.33 × 9.03  
0.16
5.42
87.2
Good, not



[0.9 mm]





uniform


8/25-28/
6M
0.6
26 × 10
0.19
4.49
84.34
good



[0.8 mm]
[.36-.85]







10/9-10/
6M [Std] 
3.8

0.044
21.9
96.4
Very









flexible,









excellent


10/9-10/
6M [skin]
1.03

0.058
16.4
95.0
Very









flexible,









excellent


4/6-8/
2M
1.2
30 × 8 
0.061
15.84
95
good


4/13-16/
2M
0.44
28 × 11
0.105
8.69
91.26
Good,









flexible


4/14-16/
6M
0.52
 26 × 8.8
0.16
5.26
86.2
Good,









flexible


4/21-23/
6M
0.89
29.6 × 10.6
0.087
10.66
92.73
Good,









flexible


5/26-27/
2M
0.04
28.3 × 10.5
0.59
0.86
51
Excellent



[Gossamer]





mechanical,



[micro-





flexible,



thin]





transparent


6/1-3/
2M
0.02 

0.76
0.48
45.0
Excellent



[Gossamer]
[0.007-0.047]




mechanical, 



[7-40





flexible,



micron]





transparent,


6/30-7/2/
2M [std] 
2.1

0.075
12.5
94.0
Flexible,




[average]




very good


7/7-10/
2M [std =
3.9
30.23 × 9.2 
0.022
44.12
98.2
Excellent


*Higher
2.5 mm]





very


pressure, low






white, no


temp [higher






shrinkage


density of sc









CO2]









7/14-18/
2M [std] 
4.0
29.6 × 8.8 
0.027
36.2
97.8
Spectacular


8/26-28/
2M
0.3

0.069
13.66
94.25
Very


* Tacky, self






flexible,


adhesive,






excellent,


static cling






repeat









9/2-4/09









[tacky],









static cling


9/21-23/
2M
0.5
  27 × 11.1
0.044
21.9
96.3
Tacky


*self adhesive,






flexible,


static cling






self









adhesive


10/7-9/
2M
0.5
27.4 × 11  
0.026
37.63
98.0
Static


*self adhesive,

[±0.05]




cling,


static cling






excellent,









very soft,









tacky feel,









sticks to









objects
















TABLE-5







Examples of Intrinsic Radiometric Characteristics 


of some Polyethylene Aerogels















Remarks on 


Preparation
Polymer
Thickness
Density
Radiometric


Date
MW
mm
g/cc
characteristics





10/7-9/*
 2M
0.5
0.026
Excellent, Best ever


7/ 7-10/*
 2M
3.9
0.022
Very good, Best to date


10/15-19/*
 2M
4.0
0.417
Good improvement


9/11/**
 2M
4.2 
0.27 
Not good




[3.1-5.3]




9/11-14/**
300K
6.0
0.097
Not good


10/21**
300K
10.0
0.096
Not good





*Anti-solvent treatment applied;


**Anti solvent treatment NOT applied.


Radiometric Characteristics include: Absorption Coefficient, α /unit length; Back Scattering Coefficient, 3; Forward scattering, 6; Reflection %.













TABLE-6







Density (g/cc) of supercritical Carbon dioxide at Various 


Temperatures and Pressures Derived from CO2 


Density-Pressure Phase Diagram. [Maximum Density


of Supercritical CO2 = 0.80 g/cc]












Pressure,







Bar [PSI]*
32° C.
37° C.
40° C.
47° C.
57° C.





80 [1180]
0.52
0.29

0.23
0.20


90 [1327.5]
0.68
0.60

0.38
0.22


100 [1475]
0.76
0.68
0.632
0.52
0.32


110 [1622.5]
0.78
0.71

0.56
0.40


120 [1770]
0.79
0.76

0.60
0.49





*1 Bar = 14.75 PSI. [Density @ 31° C. & 1070 psi [72.5 Bar] = 0.469 g/cc.













TABLE-7







Polymer Weight Loss during Gelation Reactions of


Polyethylene [2M] Gels















Initial







Polymer
Final
Dry



Reaction
Polymer
solution 
polymer
Aerogel


Preparation
Time,
weight loss
Conc.,
solution
Density,


Date
Minutes
Wt %
Wt%
Conc., Wt%
g/cc















5/14-15/
10
29.0
1.6
1.14
0.27


5/27/
10
20.0
1.6
1.28
0.4


6/2-4/
30
54.4
1.6
0.73
0.124


6/9-11/
30
61.4
1.6
0.62
0.11


6/16-18/
60
64.5
1.6
0.57
0.079
















TABLE 8







Preparation of 2M MW Polyethylene Aerogels















Initial
Maximum








Polymer
pressure in 








concentration,
Dimensions
Aerogel
Aerogel
Pore




Preparation
wt. % 
autoclave,
Density,
Volume,
Porosity %




Date
(final)
psi
cm
g/cc
cc/g
Volume
Remarks

















8/15-18/
0.9 (0.54)
1500
3 sheets:
0.24-
23.6-40.83
96.0-98.0
Acetone treatment





23.5-
0.0485








27 × 6.67-









7.9 × 0.115-









0.2






8/23-25/
0.92
1450
4 sheets:
0.0245-
30.4-34.9
97.3-98.0
Acetone treatment





26.1-
0.032








28 × 8.0-









8.4 × 0.23-









0.27






9/13-15/
0.92
1650
26.9 × 9.8 × 
0.032
31.425
97.42
Acetone treatment





0.24






9/19-20/
1.04
1700
4-sheets
0.02
51.8
98.4
No more acetone





average;



treatments. white,









silky touch, static









cling, good









radiometric









performance


10/20/
0.95
1650-1700
36.3 × 29.3 × 
0.0195
49.17
98.3
First large sheet





0.33



12′′ × 15′′


11/8-10/
0.95
1700
32.5 × 26.7 × 
0.02
49.17
98.3
13′′ × 10.7′′ sheet





0.448






11/17/
0.95
1650 same
31.9 × 26.1 × 
0.021
47.7
98.3
Flawless sheet




day drying
0.5






11/23-24/
0.67
1700
Sample;
0.0236
41.54
98.0






5.5 × 4.9 × 









0.11






12/12/
0.67 (0.62)
1700
Sample;
0.0166
59.4
98.6
Sheet tore to





7.5 × 3.0 × 



pieces





0.134






12/13/
0.62
1700, same
Sample;
0.012
82.5
99.0
First record; not




day
2.8 × 4.8 × 



integral sheet, lots





0.19



of holes


1/04/
0.60
1600, same
Sample;
0.021
47.7
98.3
4-holes in sheet




day
6.4 × 3.8 × 









0.28






1/10/
0.58
1700, same
22.5 × 30 × 
0.013
73.5
98.9
9′′ × 12′′ sheet




day
0.35






2/17/
0.29
1700, same
Sample
0.0097
102.26
99.20
Record porosity &




day
#2;
0.014-
62-72
98.7-98.9
density





3.2 × 4.6 × 
0.016








0.28;









samples









#1, 2, 4









Encapsulation of Silica Aerogel Particles in Polyethylene Aerogel

Procedures:

    A—Silica aerogel powder: silica alcogel was made by mixing silica alkoxide, water, and hydrofluoric acid catalyst at 45° C. The alcogel was micronized with a blender and sieved on 140 mesh [105 microns]. The micronized gel particles were dried by CO2 supercritical method in an autoclave.
    B—Encapsulation in polyethylene aerogel: 5 grams of 2 million MW polyethylene powder was dissolved in 630 ml hot toluene. Silica aerogel powder [80 ml] was mixed with the hot polyethylene toluene solution and 1.5-gram benzoyl peroxide catalyst was added to form gel solution. The gel solution was poured in a mold forming a thin layer of polyethylene gal enclosing the silica aerogel particles. On drying by CO2 supercritical method, an aerogel sheet was obtained as shown in Table 9.









TABLE 9







Ultra Thin Sheets of Polyethylene (MW-2 million) Aerogel 


Encapsulating Silica Aerogel Particles (<100 mesh, 150μ) 


at 90% Volume Silica Aerogel to 10% Volume Polyethylene


Aerogel















Dimen-






Prepa-
Thick-
sions
Aerogel
Pore
Volume



ration
ness
(L × W, 
Density
Volume
Porosity



Date
(mm)
cm)
(g/cc)
(cc/g)
(%)
Remarks





8/2-23/
0.74
21 × 11
0.123
7.74
95.2
Very good,








flexible sheets








encapsulating








silica aerogel








particles









Radiometric and Electric Properties of Silica Aerogel-PE Aerogel Sheets:

These sheet wafers were tested for wide band antenna applications. The test results showed that the silica aerogel-PE aerogel wafer had a sheet resistance of 250 million Ohms per square. This is much [25 thousand to 50 thousand times] higher than previously obtained by other semiconductor resistors of 5-10 thousand ohms per square. This type of aerogel sheet wafers is very promising not only for thermal sensing the w-band and infra-red [thermal insulation], but also for its very high electric resistance [very low dielectric constant]. The silica aerogel-PE aerogel wafers also had refractive index, η, between 1.01-1.09 for the wavelength between 8-14 microns, and extinction coefficient, K, between 0.0096-0.0104 for the same wavelength range.

## Claims

1. A method for making polyethylene (PE) aerogels, which comprises the steps of:
(a) in a hydrocarbon solvent in which polyethylene is insoluble at room temperature, adding polyethylene to said solvent heated to a temperature at which said polyethylene is soluble to form a reaction mixture;
(b) adding a free-radical catalyst to said reaction mixture and holding said reaction mixture until the desired gelation of said polyethylene has been achieved;
(c) cooling said reaction mixture to about room temperature; and
(d) replacing said hydrocarbon solvent with a gas to form said PE aerogel.

2. The method of claim 1, wherein said gas is air or CO2.

3. The method of claim 1, wherein drying is conducted using supercritical CO2 in step (d).

4. The method of claim 1, wherein said supercritical CO2 is applied at a pressure at or above 110 bars at 32° C.

5. The method of claim 1, wherein said polyethylene aerogel in step (d) has a MW of between about 1 million and 6 million.

6. The method of claim 1, wherein said cooled reaction mixture in step (c) is contacted with an anti-solvent for polyethylene to replace said hydrocarbon solvent with said anti-solvent.

7. The method of claim 1, wherein said hydrocarbon solvent is toluene, xylene, or benzene.

8. The method of claim 1, which is conducted at ambient humidity of less than about 40% and a temperature of less than about 26° C.

9. The method of claim 1, wherein silica aerogel particles are added to said reaction mixture in step (a).